This invention relates generally to devices and methods for the measurement of the color and reflectance of a tooth. The invention also provides for selecting a color and reflectance for a dental prosthesis which most closely matches that of the natural tooth.
The conventional method of determining a color to use in the manufacture of a dental prosthesis is to use a number of comparison samples. The samples are held up to the patient""s teeth surrounding the location where the prosthesis will be placed and the sample most closely matching the color of the teeth is chosen by visual observation. The difficulty in obtaining the optimum color match by this method is due to several factors which include the color and intensity of the light illuminating the comparison, the experience and visual acuity of the person making the comparison, and the use of a limited number of samples.
A standard, unambiguous system for measuring tooth colors is needed. Such a system should provide for accurate and repeatable color measurements in different environments such as various dentists"" offices and manufacturers"" laboratories. High-resolution color measurement data can then be provided to prosthesis manufacturers to ensure an optimum color match between a prosthesis and its surrounding natural teeth.
A number of previous inventions useful for measuring tooth colors have been patented. Some describe apparatuses for making color measurements and others describe methods for making measurements and using the results in the manufacture of artificial teeth. Several such inventions are briefly described in the following paragraphs.
U.S. Pat. No. 5,690,486, teaches a device for detecting the color of a damaged tooth and determining a color-matched restorative material. It describes the use of multiple LEDs for successively emitting light of different colors toward a target and a light sensor for receiving light reflected from the target, all contained within a hand-held, battery powered device. The device uses a fiber optic wand to convey light between the device and the tooth.
U.S. Pat. No. 5,739,915 describes as prior art a document-scanning system having a set of RGB light sources, a self-focus lens array (SLA) comprising a single row of rod lenses, and a single-row photosensor array. In operation, the light sources of each color are sequentially illuminated and light reflected from the document is focused by the lens onto the photosensor array. A set of electrical signals corresponding to each element of the array is produced by the array for each of the RGB colors. The RGB signals are then combined for subsequent color reproduction.
U.S. Pat. No. 5,838,451 discloses an apparatus for the measurement of spectral reflectance or spectral absorbance of an object or a material. The reflectance apparatus comprises multiple LEDs surrounding a photosensor, all mounted on a common substrate, and a lens for coupling light to and from the object.
U.S. Pat. No. 5,844,680 also discloses an apparatus for measuring and analyzing spectral radiation. It discloses three embodiments comprising: (1) multiple LED light sources with a single sensor, (2) a single light source with multiple sensors, and (3) multiple sources and sensors.
U.S. Pat. No. 5,851,113 discloses a system comprising a probe containing a plurality of optical fibers connected to a color measurement system. The patent describes various means of color measurement, including multicolored light sources (red, green, blue=RGB), and various analysis techniques.
U.S. Pat. No. RE 31,290 describes an imaging system comprising a rectangular photosensor array and a lens contained in a small probe connected to the system by a fiber optic bundle. Three light sources produce red, green, and blue light in sequence. Light is conducted from the sources through the fiber optics to the probe tip and reflected from the object of interest back through the lens to the array in the probe.
U.S. Pat. No. 5,766,006 teaches a system comprising an intraoral camera connected to a shade analyzer subsystem. The patent discloses a camera comprising either a single or a triple charge coupled device (CCD) array for capturing an image of the tooth in three (RGB) colors.
U.S. Pat. No. 5,760,929 discloses an image processing apparatus for discriminating colors and color patterns or boundaries in an RGB image signal and for generating signals indicating the color boundaries.
U.S. Pat. No. 3,986,777 describes a tristimulus colorimeter which measures the red, green and blue light reflected from a sample. The sample is illuminated through a probe by a light source having a satisfactory uniform distribution of light over the visible spectrum. Light reflected from the sample through the probe passes through a rotating color filter wheel, which is synchronized with a digital voltmeter, to a photosensitive diode.
U.S. Pat. No. 4,096,217 describes a method of using a tristimulus colorimeter in making artificial teeth.
Each of the preceding examples describes technology, apparatuses or methods for making color measurements or comparisons. However, none of them describes the invention of a small, hand-held and inexpensive colorimeter which can permit an operator unskilled in color analysis to quickly and objectively make consistent color measurements and comparisons.
An object of the present invention is to provide a high-resolution colorimeter system, utilizing solid-state opto-electronic technology, for measuring and characterizing tooth and prosthesis colors.
The invention is a colorimeter especially suited for dental applications comprising a hand-held probe, similar in size to a dental drill, attached by an electrical cable to a small display module.
The colorimeter provides the capability for measuring the colors of a number of points along a line on the surface of an object such as a tooth. A measurement is made while placing the tip of the probe against, or in close proximity to, the surface of the object. The display module to which the probe attaches contains a microprocessor and provides a control, display and data interface to the operator. The display module can be adapted for fastening to the wrist of the operator thereby leaving both hands free to manipulate the probe and other tools. The colorimeter is particularly well suited for measuring the color of teeth in a dentist""s office in preparation for making dental prostheses which accurately match the color of natural teeth. The colorimeter generates from a single measurement an array of color data points measured along a line on the surface of an object. From those data points, the processor can perform statistical analysis yielding a single color value, generate and display a color profile along a line, compare measured values with a preloaded table of values, or upload color data to a remote location for laboratory or manufacturing purposes. The colorimeter can also use variations in the color values measured along a line to identify boundaries of areas on a surface. For example, the color profile can be used to identify the gum line on a tooth.
As used herein, the term xe2x80x9ccolor valuexe2x80x9d means any representation of a measured color. For example, it can be a single number or a symbol, or it can be a group of numbers or symbols such as three RGB ratios, a set of tristimulus values, or a set of statistical parameters representing a vector. A color value can also be represented by the result of a comparison of measured color values to stored color values.
The probe comprises multiple light emitting diodes (LEDs) for successively emitting light of different colors toward a surface, a linear array of light sensors for receiving light reflected from the surface, and a lens for directing light from the target to the array, all contained within the probe itself.
The LEDs are arranged in a line near the probe tip and coupled to the target via a light pipe. The LEDs emit preferably threexe2x80x94red, green and blue (RGB)xe2x80x94primary colors. The three colors are preferably discrete in the sense that their wavelengths do not overlap. One or more LEDs of each color may be used depending on the efficiency of the LEDs of different colors. Additional LED colors may be used for greater accuracy if needed.
The lens is preferably a flat wafer similar in shape to a thin cross-section of a conventional lens. The lens focuses an image of the illuminated surface onto the linear photosensor array.
The photosensor array is a relatively broad-spectrum integrated circuit device which is sensitive to all the wavelengths emitted by the LEDs. In operation, the LEDs are illuminated sequentially by color and the light of each color reflected from the target is sensed by the array and represented by a vector of electrical signals.
The probe also contains circuitry for interfacing to the optical devices and for coupling the reflected light signals to the display module.
The display module preferably provides a liquid crystal display (LCD) and control buttons for operating the colorimeter. The module contains a microprocessor, memory and associated support hardware for processing, storing and displaying digital data from the probe, as well as preferably rechargeable batteries for providing power to itself and to the probe. The microprocessor can run software routines for calibrating the color measurement devices and for comparing measured values with stored values. The stored values can be used to compare or match the color of dental prostheses to the measured colors of natural teeth. The processor can be used to detect color boundaries such as a gumline by analyzing the spatial profile of data obtained from the linear array.
The colorimeter may also comprise a cradle for storing the probe and display module when not in use. The cradle may provide a battery charger and the capability of uploading or downloading data from a remote location via cable, infrared (IR) or radio-frequency (RF) links. The cradle may also provide references for calibrating the colorimeter.
The LEDs are strobed, one at a time, and light from each is reflected from a tooth and received by a linear photosensor array. The photosensor array produces an electrical analog signal which is applied to an analog-to-digital converter (ADC). The ADC converts the analog signal to digital values representing the reflected light intensity. The digital values for each color are stored in a storage device. The stored values are processed by a microprocessor as necessary to specify the color of a prosthesis which will match the color of the tooth. When the colors of all areas of interest on a tooth are measured, the results can be compiled and sent to a custom prosthesis manufacturer. Alternatively, the results can be compared with a look-up table of standard color values stored in the colorimeter""s memory. In the latter case, when a match is found this standard color identification number is displayed on a display panel.
The colorimeter is calibrated every time that it is turned on to insure accurate measurements. Reflectance measurements of a white calibration reference are made and stored for use in processing the data when data measurements are made.
New reference data on comparison samples can be loaded into the colorimeter whenever they are available. The technique for the upgrade can either be done via modem, floppy disk or hardware.